Mobile aerial lift vehicles are in common use for lifting workman at a job site to a height where they can work on elevated structures. Typically, these vehicles are four-wheeled and have an elevating assembly mounted on the vehicle chassis for raising and lowering the work platform.
Users often have requirements not easily met by existing equipment. In particular, work must often be on uneven or unlevel ground, e.g. outside a building, inside a building before the floor has been laid, on ramps, in theaters with a sloping floor, or any other job site where the ground is not smooth and level. There are two main current solutions to being able to operate an aerial lift on these types of ground conditions.
First, if a mobile aerial lift vehicle with a relatively large area work platform that is raised and lowered vertically by an articulated or scissors type elevating assembly with the center of gravity remaining within the area defined by the vehicle, is to be used on rough terrain, the vehicle is provided with a hydraulically operated outrigger at each of the four wheels. These outriggers are independently controlled to raise the four corners of the chassis so that the entire vehicle is leveled relative to the horizon. Manual leveling of the entire vehicle is difficult and time consuming. If the ground is sloping in two directions, undesirable rocking of the vehicle will occur if all four outrigger pads are not kept in ground engagement during leveling. Further, these outrigger systems are expensive, since they require a hydraulic cylinder at each corner of the vehicle, a control valve for each cylinder, and all the inherent plumbing required for such a system.
The other type of aerial lift vehicle that can be used on rough terrain is the boom, or cherry-picker, type of lift. The vehicle can be used at an uneven ground site, with the work platform at the end of the boom being self-leveling relative to the boom and vehicle. These vehicles, however, are much more expensive than the previously mentioned vehicle with articulated lift assemblies. Also, since the work platform at the end of the boom will often be at a relatively large horizontal distance from the support area of the vehicle, the work platform must be relatively small and with a low capacity.